At present, plastic window and door frames are typically assembled from polyvinyl chloride (PVC) extruded profiles using hot plate welding technology. Typically, the corner welding process involves pressing the mitered cut ends of two profiles against a Teflon-coated heated metal plate. After the thermoplastic PVC material has melted, the heated metal plate is removed and the two ends are then pressured against each other forming a hermetically sealed welded bond. Typically, in manufacturing a four sided frame assembly either one-head, two-head or four-head welding equipment is used. For four-head welding equipment, the complete frame is assembled in one operation and, taking into account the time required for frame set-up, profile loading, corner welding, cool down and frame unloading, the total cycle time is about two minutes.
As well as being a comparatively slow process, a further drawback of hot plate welding is that a large quantity of plastic flash is created at the weld line and this plastic flash has to be mechanically removed through a process that can involve cutting, shaving and routing operations. Generally, the equipment required for flash removal is complex and expensive and the process can also damage any surface coatings applied to the extruded profiles. In addition because the plastic flash material is contaminated during the welding process, the removed waste material cannot be recycled and the contaminated material can also effect the final weld strength. Finally to order consistently achieve a square right angled square corner, the equipment incorporates elaborate and complex mechanical support systems.
Vibration welding is one commonly used method for welding together the flat surfaced end walls of two thermoplastic components. As described in U.S. Pat. No. 4,352,711, the typical vibration welding process involves one component being held firmly in place in a stationary bottom fixture while a second component is firmly held in place in a moveable top fixture. By applying pressure and moving the top fixture very rapidly, heat is generated through surface friction, in a very short period of time, that melts the two contact surfaces of components that are to be welded together and thus in addition to a short cycle time, a further key advantage of vibration welding is that minimum flash is generated so that the need for mechanical flash removal can be substantially reduced. Generally, the two plastic component parts are injection molded and this allows for flash dams and other features to be incorporated into the components. As a result, even with the limited flash that is generated, its movement and location is controlled so that it is not visually obtrusive or unsightly.
Various efforts have been made in the past to use vibration welding techniques for plastic frame assembly but without commercial success. In U.S. Pat. No. 5,902,657, issued to Hanson et al, two alternative processes are described that are specifically developed for manufacturing window and door frames. One technique uses an apparatus similar to a conventional hot plate welder where a vibratory metal plate rapidly moves back forth between the ends of two profiles. To create a welded joint, the metal plate is then removed and the two profiles are pressed against each other. As described, there are some technical issues with this process because unlike conventional hot plate welding, only a thin surface layer is heated and as a result, when the vibratory metal plate is moved away, the small amount of surface plastic material that has been melted is either removed and/or rapidly cools down so that when the two profiles are finally pressed together the welded bond formed between the two profiles is poor.
There are also some technical concerns with the second alternative process described in U.S. Pat. No. 5,902,657. With this method for a four-sided frame, two opposite sides are held fixed in position while the other two sides are moveable. The moveable sides are held in fixtures that are connected to four vibratory heads that are located at profile corner ends when directly welding together two hollow thin wall profiles. Because the vibratory head moves back and forth very rapidly, it is very difficult to accurately control the final position of the vibratory head and so consequently the thin profile walls are not correctly aligned and this results in reduced corner weld strength as well as an uneven joint line which is visually noticeable.
With vibration welding, there is typically a minimum zone of disturbance at the weld line. However, for glass fiber re-enforced plastics as described in U.S. Pat. No. 5,874,146 by Kagan et al, higher structural strengths can be achieved with a wide weld zone that allows for some of the glass fibers to orient away from the flow direction and to cross the weld interface.